Systems and methods for acquiring and characterizing time varying signals of interest

ABSTRACT

Methods or systems for identifying seismic or acoustic signals of interest originating with moving motorized vehicles or footstep movement or stationary or moving machinery. A related system for monitoring an area includes a plurality of sensing devices each comprising a frame and a piezo-electric sensor element. A monitoring device is coupled to receive information from each of the sensing devices. One method includes providing a processing chain coupled to receive signal data from a sensor device which receives the signals, including a detection stage, a Joint Time Frequency (JTF) domain stage, and a classification stage. The detection stage identifies presence of signals that emerge from the background. The JTF domain stage estimates the state of the signals of interest over time. The classification stage assesses the previously derived information to form a decision about source identity. In one embodiment, the detector stage performs detections on a single cycle basis.

RELATED APPLICATIONS

This application is a divisional of and claims priority to pendingapplication Ser. No. 13/423,607, filed Mar. 19, 2012.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

The United States Government may have certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to acquisition and characterization of data,including seismic information. More particularly, embodiments of theinvention enable determination of signals of interest with reduced delayrelative to the time of signal detection and with improved reliability.

BACKGROUND OF THE INVENTION

Signal monitoring continues to be a field of great importance in orderto provide improved responsiveness in a variety of time criticalcontexts. For example, early warnings of events which may cause naturaldisasters can provide essential time for evacuation or emergencypreparedness. It is also desirable to detect the presence of human,animal or equipment activity with a low error rate in order to counterthreatening activities including military operations, border intrusionsand trafficking of illegal goods.

In the past it has been commonplace to employ multi-modal sensingschemes to characterize such activity in an automated or quasi-automatedmanner. For example, it is conventional to employ a combination ofsensor systems to discriminate certain sources from others. In oneimplementation there may be acquisition of temperature, infrared data,magnetic sensing which, in combination, can be used to confirm thepresence of a specific object such as a type of air craft or terrestrialvehicle. Such systems are complex and often not portable due to size andweight. They are not well-suited for rapid deployment and, generally,consume levels of power that make long term battery powered operationimpractical. Such objects of interest have also been identified on thebasis of data matching wherein the source, e.g., a moving motor vehicle,is known to have a generic signature. Acquisition of time varying powerdensity and spectral data associated with specific sources of seismic oracoustic energy can be compared with a fingerprint template for aspecific vehicle type to determine whether the vehicle is a motor cycleor a truck. Due to the varied nature of signatures within a category(e.g., moving trucks), such fingerprint matching techniques may have anunacceptably high rate of false detections or may result in error, i.e.,a failure to identify a vehicle as being in a suspect class. There is aneed to provide systems and methods which enable rapid detection ofspecific types of sources with high levels of confidence.

SUMMARY OF THE INVENTION

In accord with a first series of exemplary embodiments according to theinvention, there is provided a method for identifying seismic oracoustic signals of interest originating with moving motorized vehicles,comprising providing a processing chain coupled to receive signal datafrom a sensor device which receives the seismic acoustic signals, theprocessing chain including a detection stage, a Joint Time Frequency(JTF) domain stage, and a classification stage, wherein the detectionstage identifies the presence of seismic acoustic signals that emergefrom the background over time, the JTF domain stage estimates the stateof the signals of interest over time as a function of frequency and theclassification stage assesses the previously derived information to forma decision about identity of the source of the signals of interest. Inone embodiment, the detector stage performs detections on a single cyclebasis. In another embodiment, an estimate of an expected backgroundnoise is derived from time averaged data. In another embodiment, theprocessing chain separates the signals of interest from uninterestingsignals present and then classifies the signals of interest. The methodmay also include application of a routine to process time-frequencytrack information wherein a Track Set Similarity Metric is applied todetermine associations among tracks identified in the Joint TimeFrequency (JTF) domain.

In another series of embodiments, a method for identifying seismic oracoustic signals of interest originating with footstep movement includesproviding a processing chain coupled to receive signal data from asensor device which receives the seismic acoustic signals. Theprocessing chain includes a detection stage, a pattern chaining stage,and a classification stage. The detection stage identifies the presenceof impulsive transient signals of interest. The pattern chaining stageassociates like impulsive transient detection objects as a function oftime. The classification stage assesses the previously derivedinformation to form a decision about identity of the source of thesignals of interest. The detection module may include one or morecorrelation filter processes. The detector stage may perform detectionson a single cycle basis. The method may also estimate an expectedbackground noise derived from time averaged data. The processing chainmay separate the signals of interest from uninteresting signals presentand then classify the signals of interest.

According to another embodiment of the invention, a method is providedfor identifying seismic or acoustic signals of interest originating fromstationary or moving machinery. A processing chain is coupled to receivesignal data from a sensor device which receives the signals of interest.The processing chain includes a detection stage, a Joint Time Frequency(JTF) domain stage, and a classification stage. The detection stageidentifies the presence of frequency modulated (FM) or continuous wave(CW) narrow band signals of interest. The JTF domain stage estimates thestate of the signals of interest over time as a function of frequencyand the classification stage assesses previously derived information toprovide a decision about identity of the source of the signals ofinterest. In one example, the detector stage performs detections on asingle cycle basis. In one other embodiment a Track Set SimilarityMetric is applied to time-frequency track information to providecharacteristics or associations among multi-cycle detection objects foruse in the classification stage. An estimate of expected backgroundnoise may be derived from time averaged data. The processing chain mayseparate the signals of interest from uninteresting signals present andthen classify the signals of interest. The method may apply a routine toprocess time-frequency track information wherein a Track Set SimilarityMetric is applied to determine associations among tracks identified inthe Joint Time Frequency (JTF) domain.

In still another series of embodiments, there is provided a system formonitoring an area for signals of interest associated with identifiedsource types. The system includes a plurality of sensing devices eachresponsive to acoustic or seismic signals. Each sensing device comprisesa frame and a piezo-electric sensor element responsive to strain inmultiple modes. The system further includes a monitoring device coupledto receive information from each of the sensing devices. The sensingdevices may process the seismic signals and generate classificationinformation therefrom, and the monitoring device may be coupled toreceive the classification information from all of the sensing devices.In another embodiment, the sensing devices may forward data based on theseismic signals to the monitoring device and the monitoring deviceprocesses the data based on the seismic signals to generateclassifications. The monitoring device may processe data based on theseismic signals to generate direction information indicative of positionof a signal of interest.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and advantages of the present invention will becomebetter understood when the following detailed description is read withreference to the accompanying drawings in which like charactersrepresent like parts throughout, and wherein:

FIG. 1 illustrates, in simplified form, an exemplary process accordingto the invention for acquiring, detecting and classifying a signal ofinterest;

FIG. 2 is a block diagram illustrating general features of an exemplarysensor system according to the invention with which the process of FIG.1 can be performed;

FIG. 3A illustrates a sensor device 32 according to an embodiment of theinvention;

FIG. 3B illustrates in cross section a view of a sensor device accordingto another embodiment;

FIG. 4A is a cut-away view of a section of piezo polymer coaxial cabletaken along the cable axis;

FIG. 4B is a view in cross section taken through the axis of the cableshown in FIG. 4A;

FIG. 4C is a perspective view of a segment of piezo polymer tape of thetype incorporated in the cable shown in FIGS. 4A and 4B;

FIG. 5A is a perspective view of an alternate design of a sensor device;

FIG. 5B is an unrolled view of a sensor element incorporated in thedevice of FIG. 5A;

FIG. 6A is a perspective view of another alternate design of a sensordevice;

FIG. 6B is an unrolled view of a sensor element incorporated in thedevice of FIG. 6A;

FIGS. 7-10 are perspective views of still other alternate designs ofsensor devices.

FIGS. 11A and 11B are partial perspective views of an embodiment of asensor system according to the invention;

FIG. 11C is an exploded view of the sensor system shown in FIGS. 11A and11B illustrating select components thereof

FIG. 12 is a simplified schematic diagram illustrating a generalarchitecture of integrated electronics of the sensor system of FIGS. 11;

FIG. 13 is a simplified schematic diagram of interface circuitry shownin the architecture illustrated in FIG. 12;

FIG. 14 is a flow chart illustrating a sequence by which signals ofpotential interest are identified;

FIG. 15 is a flow chart illustrating a sequence by which analysesleading to classifications and determinations of Signals of Interest areperformed;

FIG. 16 is a flow chart illustrating an exemplary classificationprocess;

FIG. 17 is a flow chart illustrating another exemplary classificationprocess;

FIG. 18 illustrates a system for monitoring an area according to theinvention; and

FIG. 19 illustrates a smart node processing system incorporating asensor device and array processing according to the invention.

Like reference numbers are used throughout the figures to denote likecomponents. Numerous components are illustrated schematically, it beingunderstood that various details, connections and components of anapparent nature are not shown in order to emphasize features of theinvention. Various features shown in the figures are not shown to scalein order to emphasize features of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail particular embodiments of devices, systemsand methods according to the invention, it is noted that the presentinvention resides primarily in a novel and non-obvious combination ofcomponents and process steps. So as not to obscure the disclosure withdetails that will be readily apparent to those skilled in the art,certain conventional components and steps have been omitted or presentedwith lesser detail, while the drawings and the specification describe ingreater detail other elements and steps pertinent to a conceptualunderstanding of the invention. Further, the illustrated embodiments donot define limits as to the definition of any system or method accordingto the invention, but only provide examples which include features thatare permissive rather than mandatory and illustrative rather thanexhaustive.

With reference to FIG. 1 there is illustrated, in simplified form, anexemplary process 10 for acquiring, detecting and classifying a signalof interest. The term “signal of interest” refers to a signal which isoften of a relatively a low power level, close to or embedded belowambient noise levels. If the signal is detected as data of potentialinterest, it can subsequently be classified within a distinct group ofobjects for which the process is intended to identify members. Examplesof such groups are footsteps, stationary generators, aircraft andvarious other types of vehicles. During Acquisition 12 a seismic signalis acquired with a sensor having a relatively flat response in afrequency range extending from 2 Hz or less and upward. In theillustrated embodiments the frequency range of the sampled signal (afterdigitizing) is above 400 Hz and can extend into the kiloHertz range. Inthe following examples the sensor is of the piezoelectric type,providing signal based on charge separation. The acquired signalundergoes signal conditioning 14 which may, for example, convert thecharge separation to a voltage level, filter and amplify the signal, andprovide level adjustments needed for further processing.

During Converter Processing 16 the conditioned signal is digitized toprovide a digital stream in a form suitable for analysis. The datastream provided by Converter Processing 16 is received for Analysis 18which applies a set of criteria to segments of the digitized data todetermine whether the data contain signals of potential interest. Afeature of the invention is that data frame size is specific to theprocess. For example, the length of data frames is customized for theinitial detection of signals of potential interest as well as for eachobject classification, e.g., footsteps, or vehicles. Thus the digitalstream initially generated during converter processing is laterformatted into frames prior to processing in conjunction with specificalgorithms. The frames of data may overlap with adjacent frames in atime series or may be entirely distinct from one another. For eachprocessing function the defined frame contains a set number of datavalues captured during a defined period. For example, algorithms used toclassify a motor vehicle may format the data into frames of 1024 or 2048values acquired during a period ranging from 500 to 1,000 ms. By way ofexample, a frame may contain 2048 samples of data, each corresponding toa measured level in the serial stream of digitized data. When adetermination is made that a frame of data contains a signal ofpotential interest, two separate assessments can be made as to whetherthe signal of potential interest meets criteria for classification as aSignal of Interest (SOI). A first assessment can be provided to a remotestation while data for performing a second, more sophisticatedassessment is still being acquired. Cycle times for generating firstassessments are faster than cycle times for generating secondassessments, but the first assessments can have higher false alarm ratesthan the second assessments because they are based on less data than thesecond assessments. Nonetheless, first assessments provide earlywarnings regarding likely existence of Signals of Interest (SOI's). Thefirst and second assessments are each based on characteristic featuresderived from signals of potential interest, either over a single frameor over multiple frames of data. The characteristic features comprisesets of information useful for classifying the data of interest into oneor more distinct groups of objects. The results of these assessments areused to make a Determination 20 which provides indications as to whetheror not signals of potential interest meet threshold criteria forclassification as particular types of Signals of Interest. Summarily,the Analysis 18 indicates whether the acquired data suggests one or morespecific classifications and the Determination 20 indicates that one ormore threshold criteria are met which establish a minimum confidencelevel that the assessment is correct. With imposition of predeterminedcriteria the Analysis 18 and Determination 20 result in: (a) aconclusion that a signal of potential interest is a SOI that is a memberof a particular class of objects; or (b) a conclusion that the signal ofpotential interest is not a SOL Each conclusion is accompanied by aconfidence level.

The first assessment, being based on a limited set of data (e.g., oneframe of 2048 values) and feature information derived from the data,provides a preliminary conclusion on classification. That is, aninference about SOI classification can be provided at a predetermined,minimally acceptable confidence level for the purpose of generating ashort term Warning. The Warning is useful during the period in whichother segments of data acquired by the sensor are being processed togenerate the second assessment. With the cycle time of the secondassessment spanning a time period in which multiple frames of data arecollected and additional more sophisticated metrics are calculated, thesecond assessment provides determinations at a higher level ofconfidence than the determination generated by the first assessment.According to embodiments of the invention, a sequence of secondassessments is generated as additional frames of data are incorporatedinto the analysis process with previously acquired data frames. Theconfidence level of the most recent assessments in the sequence canincrease relative to earlier assessments in the series as moreinformation is used in the later analyses. With such an arrangement andwith the setting of a threshold level of confidence, an alert,indicating the conclusion about classification, can be generated basedon the second assessment as soon as the confidence level reaches thethreshold level. However, for a given classification, the number offrames of data required to reach the threshold level of confidence mayvary based on, among other factors, signal strength and signal type.

FIG. 2 illustrates an exemplary sensor system 30 with which the process10 for acquiring, detecting and classifying a signal of interest may beperformed.

A sensor device 32 acquires seismic and acoustic data which is sent tointerface circuitry 34 for conditioning. The analog signal is thenreceived by analog-to-digital (A/D) circuitry 36 which converts thesignal into formatted frames of digital data. Data processing assembly38 receives the digitized data to perform analyses 18 and determinations20 as described with respect to FIG. 1. For both first assessments andsecond assessments the assembly 38 outputs determinations 20 as towhether the acquired data constitute a SOL Each determination is basedon (1) criteria corresponding to one or more specific classificationsand (2) a requirement that the level of confidence of the associatedassessment that a valid conclusion has been drawn exceeds a thresholdlevel. A communications module 40 is configurable to receive andtransmit the determinations 20 via a designated mode to a receiver 42such as a handheld device or a base station located at a position remotefrom the sensor 32. The communications link between the communicationsmodule 40 and the receiver 42 may be via an Ethernet connection, a radiolink or other suitable mode.

FIG. 3A is a simplified illustration of the sensor device 32 accordingto an embodiment suitable for incorporation into the system 30. Thesensor device 32 comprises at least one signal sensing element 44positioned about a support structure 46. In this example, the supportstructure 46 is, generally, a tubular body having a chamber therein. Theexemplary body shape is cylindrical although a variety of symmetric andasymmetric shapes are suitable. The sensing element 44 is of thepiezo-electric class but it is to be understood that numerous featuresaccording to the invention can be practiced with other sensing elementsincluding, in some instances, with conventional sensors such asgeosensors of the type comprising an inertial mass whose displacement ina magnetic field generates a voltage signal. According to embodiments ofthe invention the piezo-electric sensing element 44 is in the form of acable generally known as a piezo polymer coaxial cable. Such cablesensors comprise a dielectric material having piezo-electric propertiespositioned between an inner (center) conductive core and an outer layerof conductor which may be a foil or which may comprise strands of wirebraided together. An insulative layer is formed over the outer conductorproviding an exterior coating. In principle, when the cable is deformed,e.g., due to a strain or compression, a charge separation occurs inproportion to the level of strain or compression. Circuitry coupled tothe two conductors, i.e., the inner core and the outer conductive layer,detects the charge separation as a voltage.

Piezo polymer coaxial cable has been available in multiple designs for avariety of uses such as weigh-in-motion applications. The inner (center)conductive core may comprise wire strands and the outer layer may bebraided conductor, e.g., copper. The dielectric material havingpiezo-electric properties may be a polymer which is drawn or extrudedover the conductive core, or the dielectric material may be in the formof a tape wrapped about the core in a spiral arrangement. Products ofboth designs have been offered by Measurement Specialties, Inc. ofHampton Va., USA and MEAS Deutschland GmbH located in Dortmund, Germany.Other cable designs are suitable for use as the sensing element 44 inthe sensor device 32.

FIG. 4 illustrates a segment of such piezo polymer coaxial cable, foruse as the sensing element 44, comprising an inner core conductor 52, alayer 56 of piezo polymer material formed about the conductor 52 and anouter conductor 54 surrounding the layer 56 of piezo polymer material. Aprotective insulative layer 58 is formed over the outer conductor.Conventionally, piezo polymer coaxial cables have been primarily used intwo classes of applications which each differ from embodiments of theinvention having the sensor coupled directly to seismic-acoustic wavefields in order to detect seismic-acoustic signals. In embodiments ofthe invention direct coupling between the sensing element 44 and thewave field is the predominant path of sensing seismic-acoustic signalswhile other paths are limited or absent.

Piezoelectric materials are commonly anisotropic, meaning thatelectrical output of a material depends upon the direction of themechanical strain vector with respect to the orientation of thematerial. As a result, physical constants describing the proportionalityof the electrical output to the applied strain relate to both thedirection of the applied mechanical strain and the directions orthogonalto the applied strain. This is generally denoted with two subscriptsindicating direction of the two related quantities, such as stress andstrain for elasticity, the two phenomena being related through Hooke'sLaw. The direction of positive polarization usually is made to coincidewith the Z-axis of a Cartesian tri-axial system, which is generally thethickness direction and perpendicular to the horizontal extent for asheet material. The Cartesian coordinate system directions of X, Y, andZ are usually represented by the subscripts 1, 2, and 3 respectively, asshown in FIG. 4C and those directions may be intimately related to theunderlying orientation of molecules within the material depending uponthe manufacturing process. In sheets or tapes of piezo polymers such aspolyvinylidene flouride (PVDF) manufactured to a uniaxial standard, theX or 1 direction is tightly controlled and is usually taken as the longaxis of a sheet or tape as is shown in FIG. 4C, and the axis of the PVDFmolecule is oriented along the 1 direction with polarization in the 3direction.

However, there are also PVDF sheets manufactured in a bi-axialconfiguration, where the orientation of the PVDF molecule in relation tothe 1 and 2 direction are not tightly controlled and the molecules havea more random azimuthal orientation within the plane of the film, theresult being that only the 3 direction shown in FIG. 4C (the directionof positive polarization) is tightly controlled, also corresponding tothe radial direction in FIG. 4B.

The magnitude of strain induced in a piezoelectric material for anapplied electric field is the product of the value of the electric fieldand a constant of proportionality called the piezoelectric chargeconstant, d. The piezoelectric charge constant, d, is defined for a unitvolume as the mechanical strain experienced per unit of electric fieldapplied, or alternately, the charge generated per unit of experiencedmechanical stress. Of the two subscripts, the first is usually indicatesthe direction of applied field strength, or is the polarization createdin the piezoelectric material when the electric field is zero. Thesecond subscript is then the direction of the induced strain, or is thedirection of the applied stress, respectively. Further description canbe found in Piezo Film Sensors Technical Manual P/N 1005663-1 REV E 25Mar. 2008 available from Measurements Specialties Inc., Hampton, Va.

The coefficient d₃₃ is then the induced strain in direction 3 per unitelectric field applied in direction 3, or equivalently the inducedpolarization in direction 3 (parallel to the direction of polarization)per unit stress applied in direction 3. This is radial to the axis ofthe cable shown in the axial view of FIG. 4A, and along the 3 directionas shown in FIG. 4b . The coefficient d₃₁ is then the induced strain indirection 1 (perpendicular to the direction of polarization) per unitelectric field applied in direction 3 or, equivalently, the inducedpolarization in direction 3 per unit stress applied in direction 1. Fora uni-axial film the d₃₁ coefficient is generally large, but smallerthan the d₃₃ coefficient and of opposite sign. For bi-axial films thed₃₁ coefficient is small and on the order of the d₃₂ coefficient, and ofopposite sign to the d₃₃ coefficient.

The coefficient d₃₂ is defined in a manner similar to that of the d₃₁coefficient except that the induced strain is in the 2 direction, in thesame plane as the 1 direction, but perpendicular to both the 1 and 3directions, or, equivalently, the induced polarization in the 3direction per unit stress applied in the 2 direction, within the sameplane as the 1 direction. For a uni-axial film the d₃₂ coefficient has amuch smaller magnitude than d₃₁ and of opposite sign to d₃₃.

The combination of the coefficients d₃₁+d₃₂+d₃₃ is known as thehydrostatic response coefficient and is known as d_(3h). Because ingeneral d₃₁ and d₃₃ are of opposite sign, and d₃₃ is the largercoefficient, d_(3h) is smaller in magnitude than d₃₃. In the case of abiaxial film, where the film is poled in the thickness or 3 directionbut no preferred direction for the 1 or 2 axis is enforced on themolecular scale, then the d₃₁ and d₃₂ coefficients are reduced inmagnitude and hydrostatic response is the preferred mode of usage.Additional information regarding equations of state and properties ofPVDF can be found in “Measurements and Properties of FerroelectricPolymers” by Furukawa, T. and T. T. Wang in The Applications ofFerroelectric Polymers, Wang, T. T., J. M. Herbert, and A. M Glass, eds.Chapman and Hall, New York. 1988.

The piezoelectric cable shown in FIG. 4 is constructed using a PVDF tapemanufactured to have a bi-axial response geometry. The response of thepiezoelectric-cable manufactured using this tape then will be maximizedfor hydrostatic response mode, corresponding to measuring fluctuationsin the isotropic strain.

In the past, piezo polymer coaxial cables have been used for a class ofdetection involving sensing of compressive forces. In these applicationsthe cable measures finite quasi-static strain. Examples areweigh-in-motion systems and perimeter monitoring. With the cable buriedunder a roadway or other ground mass, a signal is generated in the cablewhen a force is transmitted from above and through the ground medium tothe cable. When the cable experiences the force, e.g., a compressiveforce, a transient charge displacement is generated in response thereto.In the weigh-in-motion example, the force resulting from the weight of avehicle passing over the cable causes a physical compression of thecable proportional to the weight of the vehicle. In the context ofperimeter monitoring, movement of a person, e.g., footsteps, or movementof a vehicle, along or over the cable also results in a transientcompressive force which extends into the ground layer and to the cablesuch that both the ground layer and the cable experience compressionwhich generates a transient charge separation.

A second class of detection involves sensing hydro-acoustic wave fieldswhere piezo polymer coaxial cable senses acoustic signals whichpropagate large distances through water. See U.S. Pat. Nos. 4,794,295and 4,809,244 each of which is incorporated herein by reference. In suchsystems a piezo-electric cable is mechanically coupled to a transducerin the form of a cylindrically shaped mandrel. The mandrel is asemi-flexible structure which supports a primary mode of vibration inradial directions about the major axis of the cylindrical shape. Thestructure can be formed from a polymer such as polyvinyl chloride orother materials. The cylinder shape has been proposed for use inhydro-acoustic applications, i.e., hydrophones, because it hascharacteristics suitable for transmitting the water-borne acousticsignals to the cable via the d₃₁ mode. The received signal propagatesfrom the cylindrical body into the piezo-electric cable. With the cabletightly secured to the transducer to effect strong mechanical coupling,the received signal can propagate from the cylindrical body into thepiezo-electric cable. To assure efficient coupling of acoustic vibrationbetween the cylindrical body and the cable, in hydro-acousticapplications the cable is bound to the cylindrical body. In one designthe cable is tightly wound against an exterior surface of thecylindrical body. In other designs the cable may be pressed against thecylindrical body with an overlaying layer of shrink-wrap plasticmaterial. A result of the mechanical designs used in hydro-acousticapplications, is that the d₃₁ response of the cable is enhanced and thehydrostatic response of the cable, i.e., the d_(3h) response, issuppressed via suppression of the d₃₃ response which corresponds to theradial response within the cable. See FIG. 4.

Design of the sensor device 32 results from recognition that, instead ofreceiving signal from a transducer element distinct from thepiezo-electric sensing element 44, the element 44 couples directly withthe seismic wave field. To effect this arrangement, instead of requiringthat the cable be mechanically coupled to a rigid transducer element,such as a cylindrically shaped wall, the sensor element is thetransducer and, at most, the sensor element is only secured to establishstable positioning of the element along the frame 44 in accord with achosen configuration of the cable, e.g., a spiral geometry as shown inFIG. 3A. It is recognized that in many field applications of the sensordevice, e.g., underground positioning, the environmental conditions maydemand a rugged and durable attachment of the element 44 to the frame 46in a manner which incidentally results in some mechanical couplingbetween the element 44 and the frame 46. By minimizing mechanicalcoupling of the cable with the frame, the predominant means forstimulating the cable with seismic energy is through direct coupling ofthe cable with the seismic-acoustic wavefield. Ideally, the mechanicalcoupling may be minimized or eliminated by eliminating direct contact ofthe sensor element 44 with the frame 46. This can be effected byimposing an intermediate layer between the element and the frame wherethe intermediate layer has relatively low stiffness and relatively poortransmission properties in the frequency range of interest. As oneexample, the intermediate layer may be an open cell foam material.Further, to assure stability of cable positioning, the cable can betethered to the frame with filament under minimal tension. See FIG. 3Bwhich illustrates a version of the sensor 32 having a frame 46 ofcylindrical shape with the sensing element 44 wrapped thereabout in aspiral configuration. FIG. 3B is a view in cross section taken throughthe central axis of the frame and along one turn of the spiral cableconfiguration. An intermediate layer 60 is interposed between anexterior surface 62 of the frame 46 and the sensing element 44. A seriesof filaments 64 are secured along the surface 62 and extend through theintermediate layer 60 to the cable sensing element 44. The filaments aresecured to the sensing element in discrete places, e.g., by looselytying the filaments around the cable or by bonding the filaments to theexterior layer of the cable.

With the sensing element 44 functioning as the transducer, there is norequirement for a transducing structure that initially receives thesignal for subsequent transmission to a sensing element. In thedisclosed embodiments, the role of the frame 46 can be limited toproviding minimal support and the frame need not provide a significantpath for propagation of the seismic energy to the sensing element 44.Instead, the predominant path, and preferably the only path, of theseismic energy into the sensor device is directly from the seismicwavefront into the sensing element 44. According to embodiments of thesensor device 32 incorporating a frame, the function of the frame is toprovide necessary support to position the cable sensing element 44 in astable configuration for field deployment. Suitable designs do notimpose significant constraints on responsiveness of the cable to thewavefield in any of the vibration modes, d₃₁, d₃₂, and d₃₃. Embodimentswhich result in the hydrostatic mode, d_(3h), appear to provide greatersensitivity and frequency responsiveness relative to designs where thed₃₁mode is predominant.

In summary, the sensor device 32 is a strain sensor providing an outputsignal in proportion to the strain experienced. The piezo-activeelement, e.g., the layer 56 of the piezo polymer coaxial cable, createsa separation of charge when the strain is experienced. That separationof charge is transformed into a suitable voltage via the interfacecircuitry 34 for further processing by the system 30. The sensingelement 44 couples directly to the seismic-acoustic wave field.Accordingly, the element 44 is associated with minimal interfering orstiffening structures in order that the strain experienced by the piezopolymer on the molecular level provides a relatively large signalresponse in the form of measurable charge separation. With the piezopolymer coaxial cable acting in the hydrostatic mode, i.e., d_(3h), itis important for the cable to be constrained as little as possible inorder for the cable to deform with impinging seismic energy. Thusbonding the cable or tightly wrapping the cable to a mandrel would bedetrimental to sensitivity of the cable response in the d_(3h) mode. Thepurpose of the frame 46 is not that of providing a barrier or constraintto a particular mode (e.g., d₃₂) or to enhance another mode, e.g., d₃₃,but rather to provide integrity to the sensing element 44 and electroniccomponents which may be integrated within the sensing device 32.

Further, the sensing element is designed as a point receiver having asubstantially omni-directional response. The element 44 is designed as apoint sensor to avoid cancellation effects among components of signalacquired over the length of the cable. The sensor element behaves as apoint sensor relative to the wavelength of the seismic waves beingsensed. This is to be contrasted with use of piezo polymer coaxial cablefor perimeter monitoring, i.e., intrusion detection, where the cable isextended along a large distance (e.g., 10 to 100 meters) such thatactivity along the length of the cable results in signal which cannot beassociated with a specific position along the cable. With a straintransferred to a portion of the cable (e.g., in response to the weightof a footstep), portions of the cable not under compression act as apassive capacitance over which the charge is distributed. Impingement ofa seismic wave along a cable extending such a distance results incompressive strains on some sections portions of the cable anddilatational strains on other sections of the cable. When this occurscharge separations of opposite sign are generated in the same conductorwhich results in cancelations which give rise to no signal or reducednet signal. The sensor element 44 is behaves as a point sensor when itextends over a limited distance consistent with the wavelength beingsensed. Consequently, when a seismic wave impinges on the element 44charge separation of the same sign are generated in all sections of thecable such that the signal components generated in different sections ofthe cable are additive.

A single sensor element 44 can be used to capture strains in alldirections and the sensor device 32 can be placed in field locations fordata acquisition without regard to its orientation, or changes insensitivity and fidelity. Further, in the context of signal acquisitionfrom seismic-acoustic wave fields, the sensing element has asubstantially flat frequency response ranging at least from 1 Hz toabove one KHz. The observable frequency response may, however, belimited by characteristics of other components in the system 30, such asthe interface circuitry 34 and sampling frequencies of theanalog-to-digital circuitry 36.

FIGS. 5 illustrate an alternate design of the sensor device 32. As shownin FIG. 5A, the frame 46 has a cylindrical shape extending along acentral axis X. The sensing element 44 in the form of piezo polymercoaxial cable is wrapped about the surface 62 of the frame 46 in aserpentine design. This configuration is further illustrated in theunrolled view of FIG. 5B wherein the surface 62 is transformed into aplane for viewing as a sheet extending from reference line A-A toreference line B-B shown in FIG. 5A. The serpentine design may bemodified to include two or more separate segments of piezo polymercoaxial cable each mounted on the surface 62 for support. By way ofexample, each segment of cable may cover a 180 degree segment of thesurface 62. In another example, the separate segments of cable may beinterwoven so that each pattern extends substantially around the entireframe 46 as shown for one sensing element 44 in FIG. 5A.

FIG. 6 illustrates an alternate design of the sensor device 32. As shownin FIG. 6a , the frame 46 has a cylindrical shape extending along acentral axis X. The sensing element 44 in the form of piezo polymercoaxial cable is wrapped about the surface 62 of the frame 46 in aspiral design. This configuration is further illustrated in the unrolledview of FIG. 6B wherein the surface 62 is transformed into a plane forviewing as a sheet extending from line reference A-A to reference lineB-B shown in FIG. 6A. The spiral design may be modified to include twoor more separate segments of piezo polymer coaxial cable each mounted onthe surface 62 for support. The cable segments may be configured as adouble helix, e.g., interwoven, arrangement. By way of further example,each segment of cable may cover a 180 degree segment of the surface 62.

The sensing element 44 may also be formed about a cylindrical frame 46having a series of stand-offs circumferentially positioned about thesurface 62 of the frame. See FIG. 7 which illustrates such anarrangement for a sensor device 32 where a series of stand-offs 68extend along the surface 62 in directions parallel to the central axis Xof the frame 46. With the sensing element in the form of one or moresegments of piezo polymer coaxial cable, the stand-offs decouple thecable from the frame 46. The standoffs may comprise polyethylenes,elastomers, closed or open cell elastomers or foams or similar elementswhich facilitate decoupling of the cable from adjoining materials sothat the cable is relatively free to respond to stimulations in allmodes. The cable may be bonded at discrete points to two or threestand-offs per revolution of the spiral configuration about the frame46.

FIG. 8 illustrates another alternate design for a sensor element 32 withthe sensor device 44 configured about a cylindrical frame 46 having aseries of fluted openings 70 circumferentially positioned about thesurface 62 of the frame. The openings 70 extend along the surface 62 indirections parallel to the central axis X of the frame 46. With thesensing elements 44 in the form of one or more segments of piezo polymercoaxial cable, the openings 70 decouple the cable from the frame 46. Thecable may be bonded at two or three discrete points along the surface 62per revolution of the spiral configuration about the frame 46.

Next referring to FIG. 9, the sensing element 44 of the device 32 mayalso be formed about a frame 80 formed with two spaced-apart plates 82each having a sprocket configuration wherein a series of teeth 84,serving as supports, are formed along the periphery thereof. The platescan be structurally connected with studs, bolts or other fasteners (notshown) which connect one plate to the other. With the sensing element inthe form of one or more lengths of piezo polymer coaxial cable, thecable is wound about the structure by sequentially routing the cablebetween teeth on different plates while also sequentially progressingfrom one tooth to the next tooth on each plate. Other winding patternsare also contemplated. Generally, the cable is looped over the toothsupports and fixed at the teeth. The loops may overlap one another. Withsuch arrangements each cable is decoupled from the frame over most ofthe cable length.

FIG. 10 illustrates the sensing element 44 of the device 32 formed abouta two component frame 85 having a cylindrically shaped corrugatedsurface 62′ formed over a frame 46′ having the shape of a regularcylinder as shown for the frame 46 in FIG. 3A. In other designs theframe may be formed of a single cylindrically shaped component havingthe corrugated surface 62′ integrally formed therewith. The corrugationsextend in directions parallel to the central axis X of the frame 85 suchthat lines 86, corresponding to peaks of the corrugations, extendingoutward along the surface 62′, are oriented in directions parallel tothe central axis X. With the sensing element 44 in the form of one ormore lengths of piezo polymer coaxial cable, the cable is wound acrossthe peak lines 86 such that the cable only makes contact along the lines86, i.e., at discrete points along the surface 62′. Other arrangementsemploying corrugations are contemplated. For example, the corrugationscan be directions other than directions parallel with the axis X. Thecorrugations serve to decouple the piezo polymer coaxial cable from theframe.

Having described numerous exemplary embodiments of the sensor device 32it should also be understood that the frame, being cylindrical in shapeis not limited to such a geometry. More generally the frame 46 may be atube in any of a variety of shapes including multi-sided solids andstructures having conical or elliptical shapes in cross section. Thesensor system 32 detects and classifies SOI's. The system is a sealedunit which can be buried at an arbitrary depth beneath the surface ofthe ground but numerous other placements may be had, includingdeployment on a ground surface, above the ground, over water, in wateror in water-saturated earth.

The sensor device 32 passively monitors the seismic-acoustic energy atits location. When a signal of potential interest is observed, thesensor system 30 automatically processes and analyzes the signal. If thesensor determines with confidence that the source of the signal is adesignated target activity, it communicates the Determination 20regarding classification of the SOI to a remote receiver 42. Forembodiments where the receiver is a base station, the receivedinformation is converted into one or a plurality of user notifications.These could include a simple audio or visual alert, the cueing of acamera, an unmanned aerial vehicle or other imaging or sensor system,the activation of a geophysical information system display showinglocation (known as putting “dots on a map”), or any other actionrequired by a user.

FIGS. 11A-11C are a series of partial views of an embodiment of thesensor system 30 and select components. The sensor system 30 is in theshape of a cylinder in accord with the sensor device 32 of FIG. 3A. Thetubular shape of the sensor device enables integration of theelectronics needed to perform signal processing, including Analyses 18which lead to classifications and issuance of Determinations 20, allwithin the frame 46 of the sensor device 32. However, it is to beunderstood that the packaging of such a system with associatedelectronics is not limited to any particular shape or configuration orrequirement that the electronics be positioned within the sensor device.A feature of the system 30 is the ability to perform all requisiteanalyses at the location of placement so that data transmitted to theremote receiver can be limited to the Determinations 20.

To accomplish this functionality, the integrated electronics providesthe hardware platform on which the signal acquisition, processing, andcommunications occurs. Embedded software operates within the electronicsplatform and provides signal processing, communications support, andintercommunication within the internal assemblies. The above-describedmechanical housing provides structural support for the internalassemblies, protection from the deployment environment, and a mountingpoint for the sensing device 44.

To serve as an effective point receiver over the frequency range ofinterest, the frame 46 about which the sensing device 44 is wrapped isapproximately 3 inches (7.6 cm) in diameter and 12 inches (30.5 cm)tall. External fittings on the sensor include connection elements forpower and communications (wired or wireless), a pull handle, a purgevalve, a power-on indicator, and a tether connector.

FIG. 11A provides an external view of the sensor system 30. An opaquehermetic sealant 88, such as a durable silicone rubber, is applied overthe sensing device. In addition to rendering the system highly resistantto water intrusion, the sealant may be designed to assist in stabilizingthe spiral configuration of the piezo polymer coaxial cable which formsthe sensing device 44. FIG. 11B provides a view of the sensor systemwith both the sealant 88 and the piezo polymer coaxial cable removed,exposing the frame, which is shown as a transparent member formed of,for example, polyethylene Terephthalate Glycol having a wall thicknessof 0.32 inch (0.8128 cm) and an outside diameter of 3.135 inch (7.96cm). FIG. 11C is an exploded view further illustrating relationshipsbetween components of the system 30.

The frame 46 is connected between an upper end cap 90 and a lower endcap 92. The end caps, which may be formed of polyvinyl chloride, eachhave a “T” cap configuration such that portions 94 of the caps protrudeinto the cylindrical frame 46. The upper cap serves as a supportplatform for mounting of a handle, connectors, an indicator light and avent cap. The portions 94 include recesses for placement of O-rings 96therein which facilitate formation of a water tight seal between the endcaps and the frame 46. The frame 46, the end caps 90 and 92 and theO-rings 96 constitute a closed body assembly that defines a cavity 100for housing a series of interconnected circuit boards 102. The closedbody assembly is held in place with a series of fasteners 106. Opposingends of each fastener 106 are threaded for engagement into a threadedbore 110 on the lower end cap 92 and a threaded extender 112 positionedon the upper cap 90. The fasteners 106 extend through the upper end cap90 and thread into bores in lower ends of the extenders 112. Upper endsof the extenders 112 are threaded and pass through bore holes in ahandle 116 for fastening with a mating nut 118. The handle 116 issecured to both the upper and lower end caps 90 and 92.

A feature of the system is provision of a series of apertures 120 alongone or both end caps. The apertures are sealed with a flexible andhighly elastic membrane which may comprise the same composition as thehermetic sealant 88. Although the apertures are sealed, the membrane canundergo substantial elastic deformation in response to slightfluctuations in pressure within the cavity. This permits equalization ofpressure imbalances as noted below.

As the piezo polymer within the coaxial cable responds to excitationsinduced by seismic-acoustic waves, it is desirable that the piezomaterial freely undergo displacement in all directions, i.e., in thed_(3h) mode. Such slight movement of the cable can result in transfer ofa compressive force against the frame 46. To the extent the force cancause a pressure build-up within the closed cavity 100, the surface 62of the frame will exhibit a stiffness which counters the compressiveforce imposed by the cable. As pressure build-up occurs within thecavity 100, the level of charge displacement in the cable will bereduced and, in turn, the amplitude of measured signals will also bereduced. To the extent that movement of the frame under such force mightotherwise be impeded by such pressure build-up within the cavity 100,the portions of the membrane extending across the apertures 120 canundergo elastic deformation in response to slight pressure fluctuationswithin the cavity. This deformation reduces the magnitude of thepressure fluctuations which would otherwise occur, and the resultingresistive forces that can adversely constrain the responsiveness of thepiezo material are thereby reduced. In summary, to the extent thatmovement of the cable would be constrained by compression of the frameof the closed body assembly, this stiffening is relaxed. Provision ofthe apertures 120 allows for displacement of the membrane material andadjustment of pressure imbalances. When the frame undergoesdisplacement, an effect which would dampen the vibration response of thecable is mitigated by the elastic deformations of the membrane about theapertures 120. Thus the apertures facilitate the function of providing aframe 46 which is a flexible mounting point for the sensing element 44.

The upper end cap 90 includes an antenna connector 124 whichelectrically couples an antenna 126 to an output signal generated by thecommunications module 40. A tether connector 136 positioned on the upperend cap 90 provides a port for power input and an Ethernet connectionfor data transfer and software configuration. A purge valve 130 in theupper end cap 90 allows for pressure equalization between the chamber100 and atmosphere during periods when ambient pressure changes such asduring air transport. An indicator lamp 134 on the end cap 90 confirmspowered operation of the system 30.

FIG. 12 illustrates the general architecture and functionality of theintegrated electronics. Seismic and acoustic data acquired by the sensordevice 32 is received by the interface circuitry 34 for conditioning andamplification to provide an analog voltage signal to the A/D circuitry36. See FIG. 13 which illustrates the interface circuitry 34 in asimplified form comprising four analog stages, contained within threephysical operational amplifier integrated circuits (op amps).

The first stage 140 and the second stage 142, which receive the lowvoltage input from the sensor device, are low-noise, precision, FieldEffect Transistor (FET) op amps. The third stage 144 and the fourthstage 146 reside in a single low-noise, high-precision op amp. The firststage 140 conditions the sensor device output with a high-gain amplifierand feeds the second stage 142 for control feedback and the third stage144 for output generation. The first stage 1 also provides a controlfeedback connection with a pair of capacitors set to match thecapacitance of the sensor device 44. Typically, piezo cables larger inlength require larger capacitor values and piezo cables smaller inlength require smaller capacitor values. The second stage 142 functionsas a “servo amplifier” and ensures that the sensor element input onstage 1 is zeroed. This integrated feedback loop maintains signalintegrity by actively controlling the signal input to the first stage140.

The third stage 144 and the fourth stage 146 serve as output driverswith unity gain. The gain of these stages could be increased but doingsuch may require board level component changes. The third stage 144provides an inverted signal output and the fourth stage 146 provides anon-inverted signal output. The non-inverted output is input to the A/Dcircuitry 36, and the inverted output is provided to a test point on theassembly.

The A/D circuitry 36 provides a 24-bit analog to digital signalconversion with anti-aliasing and four-times (4×) oversampling. The DataProcessing Assembly 38 processes and analyzes signal data received fromthe A/D circuitry 36. The results of the analysis are provided to theCommunications Module 40 for external notifications and datatransmission.

The Data Processing Assembly 38 comprises a first Complex ProgrammableLogic Device (CPLD) 160, a front-end processor 162, a Digital SignalProcessor (DSP) 164 and a dual port Static Random Access Memory (SRAM)166.

The CPLD 160 governs the interaction of the front-end processor 162 andthe DSP and data transfers between SRAM 166 and the processors. The CPLDalso transmits clock signals originating on the DSP 164 to the A/Dcircuitry 36 and components on of the Assembly 38. The A/D sampling rateis defined by the front-end processor 162 and the CPLD 160 turns thisinto a clock signal which is fed at a normal and 4× (oversampling) rateto the A/D circuitry. The CPLD 160 also simulates hardware functions viaembedded software using, for example, the VHDL programming language. TheA/D circuitry 36 provides a serial feed of data to the CPLD 160. Thedata stream from the circuitry 36 begins with a unique set of bits tosignal an incoming sample, followed by 24 bits of sampled data, followedby a trailing set of bits to signal the completion of the data.

Data received by the CPLD 160 is stored in the SRAM using a circularbuffer. and is accessible to both the front-end processor 162 and theDSP 164 via the CPLD 160. This memory area can retain up to 50 secondsof historical data to assist in the signal analysis process as needed.The CPLD 160 communicates with the DSP via a Serial Port interface.

The Front End Processor provides basic monitoring of signals while theDSP 164 is idle to minimize power consumption. The DSP 164 performsadvanced analysis, detection and classification of signals. The FrontEnd Processor 162 runs Finite Impulse Response (FIR) filters for sampleanalysis. After applying the filters, the front-end processor performs agross-level pre-analysis on all samples received to determine whether asignal of potential interest is present, in which case the DSP goes intoa processing mode. This methodology minimizes the DSP's “on-time” andreduces current usage to extend the battery life of the sensor.

When a signal is detected as data of potential interest, the front-endprocessor 162 directs the CPLD 160 to activate the DSP 164 and the CPLD160 issues signals that turn on power to the DSP. In response, the DSPreturns a signal to the CPLD indicating it is ready. The front-endprocessor 162 provides the memory address (pointer) to be read to theCPLD 160 and then the samples are fed to the DSP 164 for analysis. Whenthe DSP 164 finishes processing the samples, it signals the CPLD toremove the front-end processor 162 from an idle mode and resumeprocessing of information. The front-end processor 162 re-starts at ahistorical memory location (approximately five seconds in the past) toensure that no samples are missed—ensuring continuous operation. Oncethe front-end processor 162 re-starts data analysis, it requests theCPLD 160 to deactivate the DSP in order to conserve power. An exemplaryform of the DSP 164 suitable for running signal analysis algorithms forclassifications and Detections 20 is a fixed point, dual core, 500 MHz,32-bit processor and. The DSP consists of two processors (Core A andCore B), which operate independently but have access to certain commonmemory resources.

When powered up by the CPLD 160 the DSP receives historical informationstarting at the identified memory pointer. Initially the rate of datatransfer is very fast, using the maximum SPORT interface speed, but oncethe DSP completes processing of the historical data and beginsprocessing signal data on a real time basis, the transfer rate isreduced to 2,000 samples per second. The transfer rate may be increasedto 10,000 samples per second. The DSP uses the classification algorithmsto detect and classify signals of interest. Upon identification of aclassified SOI meeting confidence level criteria, the DSP signals thecommunication module 40 for external notification of a Detection 20.

Each core has assigned responsibilities, which are summarized herein. L1cache memory is provided for each core. The total memory for L1 is 100 kof which 32 k is allocated for instruction space, 4 k for short term(scratch) use, and 64 k for data. L2 memory is dual-ported andaccessible to both cores, providing space for samples and logginginformation. A mutual exclusion lock is implemented in the hardware tofacilitate core access to L2 memory. L3 memory is external to the DSPand resides on a separate SRAM chip on the processing board.

Core A of the DSP 164 initializes the operating environment for the DSPembedded software structure, handles handshaking with embedded softwarerunning on the front-end processor 162 and embedded software running onCore B of the DSP, as well as operational management of DSP Core B,communication to the Communications Module 40 and messaging relating toalerts and warnings.

Core A of the DSP 164 receives frames of data and loads them into L2memory, using a circular queuing method. The queue may be configured tosupport 10 seconds of data. Core B retrieves the frames of data andperforms the requisite analyses, detection, classification andDeterminations 20 using the developed algorithms. Upon completion, CoreB indicates whether a SOI has been identified in accord with apredetermined level of confidence and transfers this and associatedinformation to Core A via a logging mechanism. If a SOI has beenidentified which meets confidence level criteria, Core A facilitates theinformation transmission of the Determination 20 via the CommunicationsModule. After the processing is completed, the DSP requests deactivationto revert to a power savings mode.

Core B of the DSP 164 initializes support structures, reads signalsamples, and runs the classification algorithms for the specifiedclassification types, generating warnings and alerts as necessary.

The Communications Module 40 can receive the sampled data stream, alertsand warnings generated by the DSP 164 and other messages specific to theoperation of the Data Processing Assembly 38. All data flows from theDSP 164 of the Assembly 38 to a second Complex Programmable Logic Device180 which controls flow of information to local memory and fortransmittal via, for example, an Ethernet Interface 184 or a RadioInterface 186. The Module 40 includes program and data memory 188supporting functions of microprocessor 194 and storage, shown in theform of a secure memory card 190, for storing signal data. Signal datamay be uploaded via an interface 184 or 186 for transfer to a basestation for further analyses or use in conjunction with training ofclassifier algorithms. The microprocessor 194 provides control functionsfor transmission through the Ethernet Interface and also appliesalgorithms to compress signal data prior to transmission through theinterfaces 184 or 188. In addition, the microprocessor manages radiotransmission through the interface 186 to minimize power consumption,manages Ethernet communications and provides a web-enabled interface forconfiguration parameter control and adjustment. A Global PositioningSystem (GPS) receiver 196 provides timing data which is sent to the DataProcessing Assembly 38 through the CPLD 180. The timing signal receivedfrom the GPS controls the clock signals used by the Front End Processor162 and the DSP 164 and is used to assign time information to frames ofsignal data.

The flow chart of FIG. 14 illustrates the main process loop by whichsignals of potential interest are identified, after which time singlecycle and multi-cycle processes are applied to determine whether signalscan be classified as SOI's. There is a continual flow of signal datainto the CPLD 160 which is cyclically stored in SRAM 166. The Front EndProcessor 162 defines the data length of frames and acquires signal datafrom the SRAM 166 and then executes an algorithm for Detection ofSignals of Potential Interest. When a Detection is made the Processor162 performs tasks for hand-off of processing to the DSP 164, beginningwith identification of memory address information in order for the DSPto begin reading signal data from SRAM 166. The Processor 162 also sendsa signal through the CPLD 160 to turn on the DSP 164. While the DSP ispowering up, the address at which signal data is to be read from memoryis sent to the CPLD 160. Once the DSP 164 is ready the Processor 162goes into an idle state while the DSP performs analyses relating tosingle cycle classification, multi-cycle classification andDeterminations 20. Once the DSP is finished the Front End Processorresumes Detection of Signals of Potential Interest.

FIG. 15 provides a general sequence of steps by which the DSP 164performs analyses relating to classification and Determinations 20. Oncethe CPLD turns on the DSP, Core A of the DSP initializes DSP Core B andrelated storage. DSP core A sends a signal to the Front End Processor toconfirm that the DSP is operational and enables Core B for algorithmprocessing, starting with a main loop. Signal data is received from theSRAM 166 through the CPLD 160 and the samples are buffered for input tothe Algorithms. The Main Loop also runs the Dwell Sequencer which pipesdata through each of the Algorithm sets to identify SOI's correspondingto any of Target Type 1 through Target Type n. Algorithms are createdspecific to target type, but all Algorithms follow a basic series ofoperations as shown in FIG. 15. It is to be understood that thedescriptions of the Algorithms are conceptual and functional and do notnecessarily depict the sequence in which individual steps are executed.

Each Algorithm begins with obtaining individual frames of data, nextconditioning the data, computing transforms and applying detectioncriteria. If a Signal of Potential Interest is not detected the DSPhands Signal Detection back to the Front End Processor 162 and ispowered down per FIG. 14. If a Signal of Potential Interest is detectedthe DSP proceeds with Signal Feature Extraction wherein thecharacteristic features of the detected signal are consolidated in asingle cycle detection object and a multi-cycle detection object as morefully described hereafter. As the Algorithm processes through frames ofsignal data the accumulated information is buffered. When sufficientdata is acquired there is either a classification or a determinationthat there is not a SOI. If there is a classification a Warning or anAlert is sent to Core A for issuance. Although depicted as separateflows for each Target, the execution of steps of different Algorithmsmay be interleaved. For example, Core B may perform conditioning, obtaintransforms and perform detections for multiple Algorithms beforeproceeding to Feature Extraction and Classification.

FIG. 16 is exemplary of one implementation of a classification processfor an Impulsive Transient (IT) signal. Individual frames of data areread and the algorithm applies one or more correlation filters and astatistical maxima filter which examines results of the correlationfilter for maxima. A Validator examines derived series representingnoise, correlation minima and other statistical quantities to determinewhether detections are valid and builds single cycle detection objects.Single cycle detection objects are then made available for multi-cycleprocessing. The initial step is the formation of multi-cycle detectionobjects with a Pattern Chaining Algorithm which uses the informationcontained in the single cycle detection objects and expert logic toassociate suitable detection objects and extract additional information.The multi-cycle detection objects are then examined by the ITclassification algorithms (Single Cycle Classification and RegressionTree (CART), Multi-Cycle Classification and Regression Tree (CART) andif a classification is made the result goes to the Decision Manager todetermine whether to issue a Warning (based on single cycle assessment)or an Alert (based on multi-cycle assessment).

FIG. 17 is exemplary of one implementation of a classification processfor an FM/CW signal. The algorithm performs conditioning,transformations and detections to individual frames of data. Detectionis based on statistical quantities derived from current spectral powerlevels in defined frequency channels relative to previous estimates ofnoise levels. Single cycle detection objects are created from thisinformation and made available for multi-cycle processing. Multi-cycleprocessing is initiated using a state-based estimation routine in theJoint Time Frequency (JTF) Domain, incorporating time—frequency trackinformation and dynamic models of how that information evolves in time.The Track Set Similarity Metric is a determination based on associationsamong the total set of tracks and based on characteristics of specificsources. Each of the tracks is a multi-cycle detection object used inthe classification stage which comprises a single cycle QuadraticDiscrimination Analysis (QDA) and Multi-cycle Sequential Analysis. Whena signal is classified as an FM/CW SOI the Decision Manager issues aWarning or an Alert based on confidence level thresholds set in theconfiguration for the Decision Manager.

The goal of many passive monitoring systems for situational awareness orlocal area monitoring is to detect and classify signals generated by asource or activity that requires a response. These signals areconsidered rare events embedded in a variable background of noise andfinite duration signals that are not of interest. The monitoring systemmay be comprised of a single sensor device or a group of sensor devicesthat output a signal that may be converted into a continuously sampleddigital time-series. The time series is further processed with theobjectives of identifying and separating the Signals of Interest fromthe bulk of the noise and uninteresting signals present in the datastream and then classifying the signals.

In the system 30 a sensor device 32 measures the ambient strain fieldfor seismic or acoustic energy in the form of a traveling elastic strainfield generated by a specific source or activity. The measurement,processed as a time varying voltage, which is proportional to the strainrate, is sampled and converted into a digital time series. The Signal ofInterest may be one of several broad classes of signals. Those classesmay include very short time duration transient phenomena referred to asImpulsive Transient (IT) signals, narrow-band continuous phenomena oflonger duration whose character is best expressed as a frequency domainphenomena and referred to as Frequency Modulated or Continuous Waveform(FMCW) signals, or Emergent Signals (ES), mixtures of IT and FMCWphenomena that may emerge relatively slowly from the noise backgroundcompared to an IT signal. The continuous digital time series isprocessed in the form of data frames of defined time duration andadditional series of data are derived. The additional series areformulated to highlight particular characteristics that then are usedalone or in combination as characteristic feature sets. One of thosefeature sets is generally a quantity used as a detection statistic for aparticular SOI. The calculated signal to noise ratio is an example ofthis where the expected noise, based on time averaged noise data, isused in the ratio denominator. Generally, the additional series are usedto determine presence in the data of characteristics associated withclasses to which Signals of Interest are associated.

The data frames may be sequential with no overlap of time signal data orthey may be overlapping time frames. Frames comprising 4096 or 2048signal values may, for example, be formed in sequential steps with anoverlap of 1024 or 512 or 256 values. Thus the overlap may varydepending upon processing required to identify distinguishing sourcecharacteristics of a particular signal type in the given time series.Once a signal of potential interest is detected, the combined set ofcontinuous raw and derived series data is discretized and consolidatedinto a detection object containing the characteristics of the signalwhich resulted in the detection. At this point the continuous datastream has been reduced to a set of information contained within thedetection object. Single cycle detection objects may exist acrossadjacent frames of data. The process resulting in a formation of asingle-cycle detection object is termed “single-cycle processing” andinvolves operating the detection algorithm on a continuous data stream,but the analysis may proceed in the time domain, the frequency domain,or other vector space projections of the raw time-series.

For example, it has been found that IT signals are best distinguished byapplying detection algorithms in the time-domain. Time domain processingmay be performed on data frames that do not-overlap in time at all, oron buffers that do overlap. In the context of classifying IT signals,the term “single-cycle processing” (corresponding to the aforedescribedfirst assessment), refers to the action of a set of algorithms operatingon an individual time domain data packet (e.g., one frame of data)processed by the system to generate several derived series. Single-cycleprocessing generates one or more single-cycle detection objectscomprising a consolidated set of information which describes specificfeatures of the current signal observed by the detection algorithms.

When applying the system 30 to classify an IT signal, time domainprocessing may include (i) use of a derived series to compare measuredpower of the signal with the time averaged power of ambient noise (wherevalues of noise power are estimates based on earlier data sampling, (ii)determination of the dominant frequency band of the signal, or (iii)analysis of particular time-phase patterns of the signal. For example,if the estimated power of the signal is used as the detection statisticfor a particular signal of interest (SOI), when that estimated powerrises above a specified threshold, a detection object would be opened,and remain open until a criteria of detection quality, which may be theestimated power or a feature of a different derived series, drops belowsome critical value, or a duration time is exceeded, at which time thedetection object becomes “closed.”

In contrast to IT signals, FMCW signals are best distinguished byapplying detection algorithms within the frequency domain. The system 30applies frequency domain processing on overlapping data frames with aduration dictated by the required frequency resolution and an overlapdictated by the required time resolution. In the context of classifyingFMCW signals, the term “single-cycle processing” refers to the action ofa set of algorithms operating on an individual time domain data packet(e.g., one frame of data) which has been transformed into the frequencydomain. In the frequency domain, additional information is derived whichmay include feature spaces that are normalized and reduced indimensionality or additional transforms of the frequency domain data.

When applying the system 30 to classify FMCW signals, processingincludes use of the derived series to compare the spectral power of thesignal with the time averaged power of the ambient noise and determinethe spectral content of the power structure which the signal exhibits.The result of applying “single-cycle processes” to an FMCW signal is aset of single cycle FMCW detection objects capturing narrow bandphenomena as well as the consolidated set of information which describesthe frequency content of the current time-series within the processedframe of data.

With regard to SOI's generally, the system 30 makes single cycledetection objects available to an array of information processingmodules resident in the Data Processing Assembly 38 for classifyingsignals of potential interest. Those modules may include single-cycleclassification algorithms operating on individual detection objects,multi-cycle processing algorithms operating on the extended set ofsingle-cycle detection objects, and multi-cycle classificationalgorithms operating on the combined results derived from priorapplication of information processing algorithms. The term “multi-cycleprocessing” refers to the operation of a set of algorithms on the set ofsingle-cycle results. Multi-cycle processing creates one or moremulti-cycle detection objects and additional derived information.Multi-cycle processing may include chaining or tracking algorithms,sequential analysis algorithms, expert logic, and source specificalgorithms designed to derive additional multi-cycle features. Themulti-cycle algorithms existing in the processing chain prior to theclassification algorithms generally derive additional information fromgroups of single-cycle detection objects alone or in combination withother multi-cycle information.

For classifying IT signals, the system 30 creates multi-cycle detectionobjects by associating observed time domain impulses which are thoughtto have originated from the same signal source or those which arepositioned adjacent one another in the time series. The likelihood ofthere being an association between a signal of potential interestderived from a single cycle process, and any particular multi-cycledetection object may be quantified using metrics based upon thesimilarity in power of the single cycle detection objects, thesimilarity in waveform of the single cycle detection objects, the timedistance between single cycle detections, or the expected time positionof single-cycle detections.

Monitoring the single cycle detections for association with amulti-cycle detection object may be a continuous process until themulti-cycle detection object is closed. The consistency ofdeterminations that the initial hypothesis is true may be measured overthe lifetime of the multi-cycle detection object and metrics definingthe quality of the association may be derived. An association between asingle cycle detection and a multi-cycle detection object is consideredvalid when this metric of quality exceeds some value and can beconsidered “closed” when a criteria of quality drops below some value ora duration time is exceeded. The time evolution of such a multi-cycledetection object may be estimated and predicted for following timesegments using a state based approach, such as a Kalman filter, in whichcase the multi-cycle detection object may form a chain or a cluster.

The association of a signal of potential interest derived from a singlecycle process with a multi-cycle detection object renders ITclassification a fundamentally event-driven process. When a detectionobject is adjacent the boundary of a data frame, that object ismaintained in an “open” state, and may continue to assimilateinformation present in the next data frame if the potential signal ofinterest crosses the frame boundary into the next time-adjacent frame.Another perspective of IT processing is that the multi-cycle chain orcluster is fundamentally an event-driven process. However, the desirefor computational and data-handling efficiency dictates that a cyclicprocessing be superimposed over any fundamental event-driven nature.

For the FMCW system, multi-cycle detection objects may be createdthrough the association of frequency domain detections from single timeframes (single-cycle FMCW detection objects) which are thought to haveoriginated from a single source. The hypothesis of joint-time-frequencydomain association may be based on a similarity in power of thefrequency domain detection, expected frequency value, continuation ofphase value, or similar frequency domain based features. For FMCW jointtime-frequency association, it is assumed that a given source producinga frequency domain signature will have a frequency domain feature setthat may modulate slowly with respect to the buffer rate. The timeevolution of such a multi-cycle detection object may be estimated andpredicted for the following time segment using a state based approach,such as a Kalman filter, in which case the multi-cycle detection objectforms a joint-time-frequency (JTF) domain track.

The association between single-cycle FMCW detection objects andmulti-cycle FMCW detection objects may be monitored over the lifetime ofthe JTF domain track. The consistency with which the initial hypothesisis found to be true (smooth frequency transition and smooth powertransition) may be used as a quality metric. The track is consideredvalid when this metric exceeds some value and can be considered “closed”when the quality drops below some value.

The purpose of the classification algorithms is to generate astatistically robust decision as to the type of source generating theSOI. The classifier may generate an instantaneous Determination 20 basedon whatever information is available at a point in time (singlecycle-classification), or it may generate a decision based on currentsignal information in combination with information derived from multipleprior time cycles. Initially the system 30 issuessingle-cycle-classification decisions as “Warning” determinations,reserving the final “Alert” determinations for decisions based onprocessing with multi-cycle classification algorithms. Using thisparadigm, multiple types of sources may coexist in the data stream andbe successfully separated and classified so long as the single cycle andmultiple cycle feature sets derived therein are separable. Multi-cycleclassification includes the analysis of the chain or cluster (ITprocessing), or track (FMCW processing) using statistical algorithmsthat may include linear discrimination analysis (LDA), quadraticdiscrimination analysis (QDA), logistic regression classification (LRC),and classification and regression trees (CART) among others. Themulti-cycle classification algorithms culminate in a classificationdecision based on the consolidated multi-cycle feature set.

One application the sensor system 30 is provision of a single sensordevice which operates with the afore-described analysis and reportingcapabilities to communicate a variety of information to a remotereceiver 42. In other applications, a group of independent sensors mayform a network wherein each reports to a common monitoring station,which may be a portable display device, such as a laptop computer,personal digital assistant, or a palmtop-sized personal computingdevice. The individual sensor systems may operate independently of oneanother and are typically portable, though they may be permanentlyinstalled. Both the architecture and function of the individual systemsmay be modified to suit specific applications. For example, theelectronics need not be integrated with the sensor device, theelectronics may provide different functions and the data sent throughthe communications module 40 may be varied. The system may be deployedto monitor a perimeter or length associated with valued assets, such asa war fighter forward operating base or equipment storage area, or ageopolitical border, and provide surveillance, such as monitoring foractivities of interest along a line or in an area local to theindividual sensors or network of sensors. FIG. 18 illustrates such asystem 200 comprising multiple sensors systems 30 deployed around avalued asset 202. Each sensor is deployed in the same manner as a singlesensor system 30, working independently of the others, communicating toa monitoring station 206 reports, Warnings and Alerts. Communicationbetween each sensor system 30 and the monitoring station 206 may bedirect, with or without an intervening transceiver, such as a repeater.However, communications may be through formation of an ad-hoc networkwhere the sensors themselves act as intermediaries forwardingcommunication packets to the common monitoring station 206. The ad-hocnetwork, referred to as a mesh network, is a self-configuring network ofcommunications nodes. That is, each sensor system 30 may be configuredas a communications node within the network, forwarding messages fromother sensors, while also acting as a sensor element and generatingmessages on its own.

In the system 200, information sent from each sensor system 30 is codedto identify the sending system 30. The common monitoring station 206,not necessarily located in a central part of the network, containssoftware that enables the decoding, attribution, and organization of thecommunicated messages. The common monitoring station 206 may alsoinclude a graphical interface or display, such that the information canbe referenced to the sensor identity or displayed using a geographicinformation system layout, showing the position of the sending sensor ona map. The available source signatures for monitoring in the multiplesensor embodiment are the same as for a single sensor, and include theentire variety of aforementioned signature classes. In addition, reportsof persistent activity over time are available, as are state-of-healthmessages, the assessment being made on the monitoring station usingpertinent information analysis algorithms.

For the embodiment of FIG. 18, such multisensor systems 200 aretypically provided in “kits”, containing the individual sensors systems30, batteries, embedded communications systems to link each sensor withthe monitoring station, and the monitoring station 206. When deployingthe sensors, a maximum area can be monitored by spacing the sensors atthe limits of their sensitivity ranges for the source signatures ofinterest. To increase confidence in reported Alerts, the sensors can beplaced closer together, overlapping their sensitivity ranges andproviding multiple alerts for a given signature source. Raw measurementdata are processed first by a multi-stage signal processor subsystembuilt directly inside the individual sensor systems. Further processingmay be performed by the monitoring station to draw inferences regardingSOI's.

Using a multiple sensor devices 32 or multiple sensor systems 30 in amonitoring system adds the advantages of larger area coverage and,optionally, higher confidence in alert accuracy relative to theperformance of single sensor systems. Additional advantages are gainedby forming a multiple sensor system as a coordinated array of sensordevices and using coordinated array data processing methodologies. Insuch embodiments, some of the data processing electronic elements may bephysically removed from each sensor system 30 and be contained in a“smart node” processing system. The individual sensor systems may becoupled to the Smart Node by cable having internal conductors that carrycontinuously digitized seismic data from each sensor to the Smart Node.The sensor systems may also transmit data to the Smart Node using awireless communications link.

A functional illustration of such a smart node processing system 210,utilizing array processing, is shown in FIG. 19. Multiple sensor devices32, or sensor systems having processing capability of the naturedescribed for the sensor system 30, are deployed as an array 212. Thesensors continually acquire seismic acoustic data and transmit the datato a smart node processing unit 214. In addition to receiving,organizing, and storing digital seismic data, supplying power and amaster clock signal to the individual sensor systems (for a wired arrayarrangement), and monitoring the state of health of the individualsensors, the processing unit 214 combines and processes the entirety orsubsets of the data received from the sensors populating the array 212in order to extract actionable information. The processing unittransmits the information developed through processing the combined dataeither by wire or wirelessly to a typically remote monitoring station216, which can be any computer-based system containing a transceiver andmonitor/display capability. The remote monitoring station 216 containsall of the functionality of the multiple sensor network commonmonitoring station 206, and may contain additional functionality such asa visual representation of the site's response to its environment, anear real time visual display of the processed array information, avisual display of historical information from the array, and near realtime and historical information from other independent sensors notwithin the array and other independent arrays.

Such geographical information displays appear similar to weather radar,where activity detection probabilities appear as color coded or colorcontoured regions superimposed over the geographical map of themonitored area. Such outputs from the combined information set may befused with additional visual or other systems such as pan/tilt zoomcameras and closed circuit television for security personnel assessment.This approach provides extreme scalability from a single site to a largegeographical area that may consist of multiple sites. The visualgeographical fused information display can easily zoom in from largearea coverage to specific sites of activity while providing completesituational awareness of the larger contextual monitored area.

The smart node processing system 210 performs complex andcomputationally intensive operations such as combined coherentprocessing of the array data. Coherent array processing providesadvantages by performing signal processing in both time and space. Thisrequires seismic-acoustic sensors that are synchronized to a common timebase and deployed as arrays, with multiple arrays potentiallysurrounding an area of interest. The key enabling technologies for thiscapability are (1) high sensitivity, highly coherent seismic-acousticsensors, and (2) Frequency-Wave Number (FK) beamforming. Three coherencefactors must come together to make seismic-acoustic array processingpossible: (1) coherence in time, (2) coherence in space, and (3)measurement coherence. Temporal and spatial coherence are achievedthrough Global Positioning System (GPS) technology and by laying outsensors in arrays with appropriate spacing.

“Measurement coherence” means that the sensors must respond identicallyto seismic energy, in both amplitude and phase. Most seismic sensors usesystems of springs and masses in conjunction with damping elements toconvert seismic energy to electrical voltage signals, and as such thesesensors are difficult and expensive to manufacture with sufficientcoherency for array processing, especially over a broad frequency bandthat includes frequencies in the audible range acoustic energy. Thedisclosed sensor, due to its design and sensing material properties,provides extreme coherence from sensor to sensor.

The unusually uniform phase and frequency response between sensorsgenerates an unusually high array gain compared to typical seismicsensors when coherent processing methods are employed. Coherentprocessing, often called beamforming but not limited to creating beameddata, is commonly used with active and passive RADAR and SONAR systemsin order to “spotlight” particular geographic sectors through theamplification of coherent signals and attenuation of incoherent noise.Seismic array processing uses the same well-established principles toenhance detection in a specific geographic area by defining a beam usingthe appropriate frequency and wave number parameters.

In geophysical investigations coherent processing methods are formulatedinto wave number manipulations within the frequency domain to formbeamed data (FK beams, after the generally accepted mathematical symbolsfor frequency and wavenumber). The general FK algorithm searches forglobal maxima of combined array power as a function of both frequencyand the wave number vector, where frequency describes periodicities intime (cycles per second) and wave number describes periodicities inspace (cycles per meter), determined for overlapping time frames. Theresult is a beam that pinpoints the direction of seismic energy evenwhen the energy propagation is not simple. Coherent processing isperformed by the Smart Node software, either in embedded or host-basedelectronics platforms depending upon the requirements of the situation.

Extending the FK beamforming methodology, energy maxima exceeding apreset threshold, registered as single-cycle detection objects, may beautomatically tracked using a Kalman filter or similar trackingalgorithm operating in the FK slowness space. For persistent FK domaintracks surpassing a time-cycle length threshold, a beam recipe may beformulated, consisting of a set of delays and weights, one for eachsensor element in the array, allowing either a traditional delay-and-sumfixed beam to be formed on the target, or allowing for a continuallysteered beam to be formed as the track is updated, and existing as longas the FK track persists. As the FK track is updated, the Kalmanalgorithm predicts the next location of the source, a new beam recipe isformed, and the steered beam adjusted and computed.

The resulting time series waveform becomes a “dynamic seismogram” thatfollows the source of energy wherever it moves. If the energy isstationary, the steered beam remains fixed, but ready to move if thesource moves. Steered beams appear when a source of energy appears, moveto follow the source as the source moves and disappear when the sourceof energy disappears

The time series waveforms generated from the fixed or steered beam maythen be forwarded to the existing suite of signal and informationprocessing algorithms for further processing and alert generation.

Although example embodiments according to the invention have beendescribed, numerous other devices, systems and methods will be apparent,and it will be understood by those skilled in the art that variouschanges may be made and equivalents may be substituted for elementsthereof without departing from the scope of the invention. Accordingly,the scope of the invention is only limited by the claims which follow.

The claimed Invention is:
 1. A system for monitoring an area for signalsof interest associated with identified source types, comprising: aplurality of sensing devices each responsive to acoustic or seismicsignals wherein each sensing device comprises a frame and apiezo-electric sensor element responsive to strain in multiple modes;and a monitoring device coupled to receive information from each of thesensing devices.
 2. The system of claim 1 wherein: the sensing devicesprocess the seismic signals and generate classification informationtherefrom; and the monitoring device is coupled to receive theclassification information from all of the sensing devices.
 3. Thesystem of claim 1 wherein: the sensing devices forward data based on theseismic signals to the monitoring device; and the monitoring deviceprocesses the data based on the seismic signals to generateclassifications.
 4. The system of claim 3 wherein the monitoring deviceprocesses data based on the seismic signals to generate directioninformation indicative of position of a signal of interest.
 5. Thesystem of claim 1 wherein the monitoring device processes data based onthe seismic signals to generate direction information indicative ofposition of a signal of interest.